1. Field of the Invention
The invention relates to AC to DC converters, and more particularly to an AC to DC converter having an improved power factor.
2. Prior Art
The conversion of AC energy to DC energy is typically accomplished by either passive or active means.
In the passive approach the AC input voltage is rectified and inductors or capacitors are provided to filter the rectified voltage to a DC level with preferably minimum AC content. This approach is simple and uses a minimum of components. The disadvantage is that the input current drawn from the AC source is severely distorted and does not have the shape of an ideal sine wave. As a result there is a relatively low power factor and voltage distortion occurs in the AC source when source impedance is present.
The active approach also entails rectification of the AC voltage. An active converter is then used to draw input current in proportion to the rectified input voltage. The current into this converter is controlled with a feedback loop that uses the rectified voltage as a reference. The input current is very nearly a perfect sine wave in phase with the input voltage. This technique can produce a power factor of near unity and input current distortion under 3%. The drawback is that it requires a complicated control scheme, which utilizes either many discrete components or a specialized control integrated circuit. The response of the control loop needed for proper power factor operation, may also interfere with other loop requirements. The DC output voltage must also be greater than the peak of the input AC voltage, and in some applications it is desirable that the DC output voltage maintain a lesser value.
In many airborne applications of the invention, the electrical systems are supplied by a 400 Hz 115 VAC source. This voltage then requires conversion in an AC to DC converter to a DC voltage of 100-170 volts, which is within the range of operation of conventional semiconductor power devices. The final element in the power train is a DC to DC converter that produces a reduced voltage usually at set voltages between 5 and 28 volts at proportionally higher currents.
The present AC to DC converter has application to such a power train. In such an application, it is essential to electrical efficiency that the power exchange between the elements of the power train be efficient. The 115V 400 Hz AC source has a limited power capability and significant internal impedance. In the interest of electrical efficiency, it is important that loads, of which the AC to DC converter would be one, draw current with a near unity power factor and with minimal harmonic distortion.
The AC to DC converter of the present invention has immediate application to aircraft systems. In such systems, in addition to electrical performance requirements, the supplies should be of minimum weight and minimum bulk.